MCQ Detailed View

Grignard reagents are highly reactive organometallic compounds of the general formula R–MgX, where R is an alkyl or aryl group and X is a halogen (Cl, Br, or I). They act as strong nucleophiles and bases. When a Grignard reagent reacts with ethylene oxide (an epoxide), an important reaction occurs that extends the carbon chain by two carbon atoms.

The reaction takes place in two steps:

1. 
   

   Nucleophilic attack on ethylene oxide:
   The carbon atom of the Grignard reagent (R⁻) attacks one of the electrophilic carbons in ethylene oxide. The strained three-membered ring of ethylene oxide opens easily. This gives an intermediate magnesium alkoxide.

   
   

   General reaction:
   R–MgX + CH₂–O–CH₂ → R–CH₂–CH₂–O⁻MgX

   
   


2. 
   

   Acid hydrolysis:
   On treatment with dilute acid (H₃O⁺), the alkoxide group is protonated to give a primary alcohol.

   
   

   Final product: R–CH₂–CH₂–OH

   
   

The important point is that the product is always a primary alcohol, regardless of the type of alkyl group in the Grignard reagent. This is because the –CH₂–CH₂–OH unit comes from the ethylene oxide itself.

Let’s review the options:

• 
  

  Primary alcohol: Correct, since the product is R–CH₂–CH₂–OH.

  
  


• 
  

  Secondary alcohol: Not possible from this reaction.

  
  


• 
  

  Tertiary alcohol: Requires reaction with other carbonyl compounds like ketones, not ethylene oxide.

  
  


• 
  

  Dihydric alcohol: Incorrect, because only one –OH group is formed.

  
  

This reaction is widely used in organic synthesis to increase carbon chain length and produce primary alcohols with predictable structures.